could not vs. might not

[Steveg]

HI,

My name is steve and is there anyone who can answer it for me that what is the difference between Could not and might not.I am confused with the usage of these two modals in negative because it is said, could not (negative) is different from May not and might not .Here is the comparison
Sarah could not have got my message .Otherwise she would have replied( is it present situation or what ?
I wonder why Sarah has nit replied to my message .I suppose she might not have got it ( same with it ? present or past ?

Looking forward somebody can help me out in this

[Alan]

Hi Steven,

If we go back to 'can' and 'may' in one of their basic meanings, we see that they both suggest possibility but 'can' if you like, is a little more likely (possible) than 'may'. In the past forms 'could' and 'might' there is the same distinction.

When we write: She could not have received my message, we indicate that really that's not possible. If we write: She might not have received my message, we indicate that there is some doubt as to whether she did or did not receive the message.

The same distinction would apply if you used 'she cannot' or 'she may not'.

Hope this helps a little.

Alan
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English as a Second Language
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[Bhupendra]

what is the difference between these & how to use; i got ,i have got